1. Field of the Invention
The present invention relates to a compound semiconductor device and a method of manufacturing the same.
2. Description of Related Art
Recently, development of an electron device has been performed actively in which, on a substrate made of sapphire, silicon carbide (SiC), gallium nitride (GaN), silicon (Si) and the like, the crystal growth of an aluminum gallium nitride (AlGaN)/GaN hetero structure is performed and a GaN layer is included as an electron transit layer. For example, a high electron mobility transistor (HEMT) is known as such an electron device. The band gap of GaN is 3.4 eV and is large compared with 1.4 eV of GaAs. Because of this, GaN is expected as a semiconductor material that can realize a high breakdown voltage electron device.
High voltage operation is demanded in an amplifier for a base station of a mobile telephone. Because of this, the HEMT used in the amplifier for a base station is required to be high voltage operation. So far, a value exceeding 300 V has been reported as the breakdown voltage when the current of a GaN-HEMT is off.
Further, the most preferable output characteristic of a GaN-HEMT is currently obtained in the case of using a SiC substrate as the substrate. This is because SiC is superior in thermal conductivity.
However, the cost of a semi-insulating SiC substrate used as a substrate of a high frequency device is very high with the reason that the control of insulation properties is difficult, etc. Because of this, there is a possibility that the spread of GaN-HEMT may be hindered if the semi-insulation SiC substrate is used. Then, as a counter-measure, a conductive SiC substrate is considered for use as the substrate of GaN-HEMT. The conductive SiC substrate is developed proactively for use in an optical device or a low frequency-high output electron device as an objective, mass production and an increase in the diameter have been already realized, and it can be obtained less expensively compared with the semi-insulating SiC substrate.
In order to use the conductive SiC substrate as the substrate of a high frequency device however, there is a necessity to secure a sufficient distance between the conductive SiC substrate and an active layer from the viewpoint of insulation properties and capacity that determine the high frequency performance. Therefore, a relatively thick buffer layer made of a high resistance crystal such as AlN has been inserted between the conductive SiC substrate and the active layer.
A hydride vapor phase epitaxy (HVPE) method etc., is used in the formation of an AlN layer used as the buffer layer between the conductive SiC substrate and the active layer. According to the HVPE method, it is possible to grow a thick AlN layer with high speed and with low cost.
However, a large unevenness is often generated on the growth surface of the AlN layer formed with the HVPE method. In the case of forming a compound semiconductor layer containing a GaN layer on an AlN layer having a surface with such large unevenness and configuring an electron device such as HEMT, sufficient device characteristics can not be obtained.
FIG. 1 is a time chart showing the transient response of the conventional GaN-HEMT using the GaN layer in which Fe is added as an electron transit layer.
Drain voltage VD, gate voltage VG, drain current ID (Fe:GaN) in the case of using the GaN layer in which Fe is added as the electron transit layer, an ideal drain current ID (ideal) are shown.
When a fixed drain voltage VD and a rectangular pulse-shaped gate voltage VG are applied, an ideal drain current ID (ideal) becomes a rectangular plus-shaped waveform corresponding to the rectangular plus-shaped gate voltage VG.
However, an interaction between defect state forming by Fe impurity and an electron occurs in the GaN layer in the case of making the GaN layer in which Fe is added to the electron transit layer. As a result, a drain current ID (Fe:GaN) becomes a waveform that decreases with time against the gate voltage VG of the rectangular shaped pulse. In such case of making the GaN layer in which Fe is added as the electron transit layer as such, a transient response is caused by defect state forming by Fe impurity, and the drain current ID (Fe:GaN) decreases with time.